1. Field of the Invention
The invention relates to aircraft area navigation systems (RNAV) particularly with regard to systems that provide vertical navigation control.
2. Description of the Prior Art
Prior Art RNAV systems provide vertical navigation on straight line paths from one altitude to another. Such systems have been restricted to fly vertical paths that are defined by a final altitude and a constant vertical flight path angle to that altitude. The termination altitudes are those assigned to waypoints, i.e. fixes utilized by the RNAV system for lateral navigation. One of the simplest conventional vertical paths is point-to-point navigation from one waypoint to another. The prior art RNAV systems connect the first waypoint with its associated altitude to the second waypoint with the altitude associated therewith utilizing a straight line, the system computing the flight path angle defining that line. Vertical navigation control is then based on deviations of the aircraft above or below this line.
Although such straight line navigation provides acceptable performance for many flight conditions, optimum performance is not obtained during particular specialized procedures. For example, it is often required during standard departure and arrival procedures that the aircraft cross defined waypoints "at-or-above" or "at-or-below" specified altitudes. On an "at-or-above" designation it is desirable that the aircraft climb to cruise altitude as quickly as possible to conserve fuel. The climb gradient varies with such factors as aircraft gross weight and atmospheric conditions. Under manual control such climbs are often flown at fixed airspeed or Mach number. However, if the procedure is to be predefined for the automatic navigation of a conventional RNAV system wherein the aircraft is restricted to fixed straight line flight paths, then the worst case angle for the heaviest aircraft must be selected thus diminishing performance efficiency for most aircraft or the pilot must manually select each waypoint altitude based on his estimate of aircraft performance which is an undesired pilotage task. Alternatively, with conventional RNAV systems that require the aircraft to fly a fixed vertical flight path angle when climbing or descending, depending on atmospheric conditions and weight, the aircraft may not be able to maintain the constant flight path angle requiring the pilot to disconnect the system and fly the aircraft manually.
Another problem in the vertical path control of conventional RNAV systems occurs when the altimeter reference is changed from local barometric pressure to the pressure altitude setting of 29.92 inches of mercury as the aircraft ascends through the predetermined transition altitude and conversely, when the altimeter reference is changed in the opposite direction when the aircraft crosses the transition altitude on descent. This change in altimeter reference normally takes place at 18,000 feet as the aircraft climbs or descends between altitudes near the ground and the cruise altitudes. Conventional RNAV systems utilizing straight line and constant flight path angle navigation experience a discontinuity in the vertical steering error when the barometric (baro) setting is changed due to the resulting change in the apparent altitude of the aircraft. This discontinuity causes the pilot to again disconnect the conventional RNAV system and manually maneuver the aircraft through the discontinuity from the one path to the other. Alternatively, the undesirable piloting technique of slowly changing the barometric reference so that the aircraft gradually transitions from one path to the other is utilized in the prior art systems.
An additional problem in the vertical steering control of conventional RNAV systems occurs when the aircraft descends from cruise altitude to the terminal area of an airport because of the necessity of the descending aircraft to decelerate from cruise speed to terminal area speeds during the descent to the airport. U.S. aviation regulatory agencies typically require a deceleration to 250 knots by the time the aircraft descends to 10,000 feet. Typical jet transports are generally incapable of effecting this deceleration on normal descent flight paths by merely reducing thrust. Since generally a minimum thrust is required to maintain cabin pressurization, the pilot conventionally reduces the rate of descent by decreasing the descent angle until the speed is sufficiently reduced and then resumes the descent. By the time the aircraft decelerates to the required airspeed the aircraft acquires a significant altitude error and is generally not capable of reducing the error to zero by the time the waypoint is reached. In order to perform this manual maneuver the pilot must disconnect the automatic flight control system (AFCS) from the RNAV system and/or ignore the flight director commands. Additionally with conventional RNAV systems that provide only straight line flight paths, an additional waypoint must be inserted at the transition altitude to permit the level flight path section for the deceleration, thus introducing an undesired complexity.